Patterned windings form in randomly wound yarn packages when the yarn is laid down several times at the same or an adjoining location on the yarn package resulting in ribbon-like yarn layers. Moreover, pattern windings always occur when the winding ratio, i.e., the number of bobbin revolutions to the double stroke of the yarn guide, equals a whole number, and there are several occasions in the course of a winding process of a yarn package when this happens.
Pattern windings considerably interfere with the later unwinding of yarn packages. For instance, because of the inhomogeneous composition of the yarn layers, it is possible for entire yarn layers to be ripped off the yarn package surface during unwinding. In order to produce a high quality yarn package, it is therefore necessary to employ methods for preventing pattern windings.
One conventional method involves the varying of the winding ratio particularly during occasions when pattern windings are known to occur. For instance, a winding machine is known from Swiss Patent 215 637 where yarn packages are driven by friction rollers and a toothed wheel gear having a preset gear ratio connects the drive of the friction rollers with the drive of the yarn guides. The circumferential speed of the friction rollers is continuously varied between a maximum and a minimum value and the number of strokes of the yarn guide is subsequently varied because of the toothed wheel gear. A disadvantage to this method is that a fixed gear is structurally elaborate and it is necessary to keep appropriate toothed wheel gears on hand for changing the gear transmission when, for example, a new roller geometry is used.
Another method and apparatus having an electronic control between drives of the winding rollers and the yarn guides is disclosed in German Patent Publication DE 25 34 239 C2. In this reference, separate drive motors are provided for driving the yarn guides and for driving the winding rollers which drive the yarn packages, with an electronic gear connecting the two drive motors. In the method and apparatus the rpm of the drive motor for driving the yarn guide is arbitrarily changed while the drawing speed is maintained substantially constant with the aid of the electronic gear.
A method for randomly winding yarn packages wherein yarn is also supplied at a constant yarn running speed is known from German Patent Publication DE 43 37 891 A1. In this method, a non-periodic change of the traversing speed is performed for preventing pattern windings. Specifically, the maximum and minimum values of the traversing speed, whereat respective switching from acceleration to deceleration and vice versa occur (the reversing points), are varied within predetermined limits by a computer.
A common drawback to the two previous methods is that each requires a large outlay in control techniques for coordinating the drives of the friction rollers and the yarn guides whereby the resultant drawing speed of the yarn is maintained substantially constant. It is therefore an objective of the present invention to provide a simplified method for preventing pattern windings.